As you know, USMS passed a rule that will require us to measure any pool in which we hold a meet, IF we want the times from that meet to count for USMS Top Ten. I'm trying to figure out the best way to do this - what sort of tape to buy, the best method to use, etc. (Can you tell that I'm an engineer?)
I'd like to hear from those of you who have measured pools. What brand of tape do you recommend? What length? (60 meters?) Any idea where I can buy such a tape? Any suggestions for how to get a good (accurate) measurement, especially over a 50 meter course?
FYI, below are the instructions on how to perform pool measurements, taken from the USMS Pool Length Certification Form.
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"A completed form must be on file or submitted with an application for sanction or recognition to your LMSC Sanctions Chairman. For pools utilizing bulkheads, completed forms from each session must be included with results sent to the Top Ten Chairman if the results are to be included in Top Ten submissions.
Measurement procedures:
--Measurements must be conducted using a measuring device with the minimum measuring quality of at least ± 0.005 m (±.0.20 inches or 0.016 feet) over the nominal distance. A laser based measuring device may be used but it must be as accurate as required for steel tapes. Tapes may not be combined to perform the measurements.
--Measurements may be reported in either the English system (feet/inches) or the metric system (meters/centimeters).
Instructions for pool length measurements:
--Measurements must be conducted for each lane. Inspect each end of the pool to insure that it is vertical. If there are any protrusions from the wall into the pool between 0.8 meters (2.5 feet) below the surface to the top of the touch pad or wall or up to 0.3 meters above the surface whichever is higher, you must use the protrusion as your measurement point. Otherwise, measurements must be taken in the center of each lane at water level.
--The measuring device shall be moved up/down and left/right at least 12 inches (unless you are on a protrusion) during each measurement and the minimum distance for each measurement recorded.
--Measurements may be conducted with or without touchpads in place. However, if touchpads are used the pool must still meet the length requirements with them in place.
--Permanent courses need only be measured once unless structural changes have occurred since original measurement.
--For pools with a moveable bulkhead, after each session a course measurement must be confirmed by a responsible person. Only the outside lanes and a single center lane need to be measured to confirm the integrity of the bulkhead and its placement."
Former Member
Originally posted by ljlete
Wayne,
I am curious as to the physics behind the inability of a laser to measure over water. The laser is a parallel beam of light with very little divergence, especially over the distances we are talking about. Therefore, there should be no reflection off of the water. At least this should be the case with the beam going from the laser to the other end. Now, maybe there is a problem if the target causes the beam to diffuse but given that the instrument averages multiple measurements and the water is not likely to be still (to the same degree as the beam is coherent) then the noise should average out. Did he/she give you an explanation?
Leo
I'll look up the technical details, I had read them about 3 years ago when my company was looking into purchasing the reflectorless laser surveying equipement.
It has to do with the way water reflects the dipersed signal, and the feedback you get to the instrument gets a 'dirty' signal back, and you can never be very certain oif the signal you read back is from the target you sighted, or from the interference.
When you get into higher precision measurements, these details matter.
If you were measuring something to +/- few inches, it wouldn't be that critical. This is just why there is a lot of laser instrumentation used in grading, because the accuracy needed is often in inches, and not thenth's of an inch, and when you need to build hardscape and buildiongs and higher precision things, surveyors come in with higher end instrumentation, which is not 'laser' based... Even though coloquially, many people tend to call instruments we surveyors use "Lasers", they're not lasers.
I'll look up the technical details and specs that come with the laser instruments, it's all written and explained in there. I just don't recall all of it off the top of my head.
Former Member
Originally posted by mattson
I don't know anything about these laser sighters, but I do know a little physics. The speed of light depends on the medium it passes through (air, glass, water, etc.). This will cause all sorts of interesting phenomena. Think of a mirage on a hot day. :cool: So it sounds like the argument is that the optical properties of the layer of air just above a body of water are different enough than the surrounding air (due to temp, humidity, etc.), that it can throw off these instruments.
Or maybe I'm barking up the wrong tree. :)
You're right, and you're preaching to the quire, at least as far as I'm concerned. It's not so much the properties of the layer above the water, but it's the fact that it's being sughted to a non-perfect target. You're trying to reflet the laser beam ioff an uneven wet surface, which is also just inches from water.
I still haven't had the time to pull out the papers that talk about the issues with laser sighting near water and on wet surfaces. I do know that when I looked into it 3 years ago when considering a purchase of a refined piece of equipement, it made it non precise enough for cetain applications, that were near water, and would not work well when the sigted surface was wet...
That' when it comes to the 'visible laser beam measurements'
Originally posted by mattson
(Connie, how would you get a more accurate distance measurement? You mentioned higher-end non-laser based instruments. I ask, because the definition of a "meter" used to be a physical bar of platinum stored somewhere, and is now based on a wavelength of light.)
The original definition of a meter was astronomically based. It was the 1/10,000 of the 1/4 of the length of the central meridian.
To generalize it is based on the earths circumference, measured along the central meridian.
The bar that was cast to that distance... it's length was based on the astronomical observations.
Later on it got redefined to the specific period of the wavelength of light.
Former Member
Oh, One more thing Mattson...
It's important that we start differentiation between the actual lasr beam and the coloquialisam 'lasers' as a generalized piece of equipement that people talk about.
When they talk about 'laser' a lot of people end up refering to the little cheap hand held distance meter, which is based on laser measurements, but which has a different grade laser beam than the high precision equipement.
There is a whole array of qualities there.
What is frequently refered to as 'laser' in my business is the low end equipement. The higher end equipement still has infrared spectrum laser based technology on it, in part complemented by radio equipement as well.
Former Member
I wish I swam fast enough for 2mm or 0.01 seconds to be meaningful!:D
Former Member
SPMA discussed this with a mechanical engineer who knew about laser measurements, and they quickly added that measurement over water was impossible. Reflections from the water supposedly ruin the accuracy. I have not looked at the laser device web sites listed, do they mention measurement directly over water?
The engineer added we should use a surveyor, only $300-$500 per pool.
SPMA has used a special steel tape that uses a tension device to determine droop and maintain accuracy. The tape is over 20 years old, and very high quality. I guess we got our monies worth out of it:p
Former Member
Well.... It's simple!
We need to get more surveyors to start swimming, then we can start twisting their arms to donate services!!!
Yeah, wouldn't it be nice ;)
Former Member
Hey Leo, where were you and your white paper when I claimed that -.00 meters meant that a pool could be 5 mm shorter than the nominal length? You remember, that time when I was called denegrating names and told that it really meant the pool could not be 0.0000000 shorter than the length?
Just a peeve, nice post.
